Knife sharpener



FIG. 1 is a front perspective view of a knife sharpener in accordance with the present design;

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines represent unclaimed portions of the claimed design and are included for illustrative purposes only and form no part of the claimed design. The shading represents contour of the claimed design. 

CLAIM The ornamental design for a knife sharpener, as shown and described. 